(a) Field of the Invention
The invention relates to a process for the recovery of sulfuric acid. More specifically, this invention is concerned with a process which involves the liquid-liquid extraction of sulfuric acid from waste industrial solutions and the transformation of sulfuric acid into a valuable product. In particular, the invention is directed to the recovery of sulfuric acid from waste acids produced in the industry of TiO.sub.2 pigments.
(b) Description of Prior Art
The major part of the sulfuric acid which is produced in the world is used as a reagent in various industrial processes. The main users of sulfuric acid are the chemical, petroleum, mining, metallurgical and steel industries. Two important consumers of sulfuric acid in the chemical industry are the fertilizer and pigment industries. At the end of many of these processes, the sulfuric acid is found as waste acid which is contaminated with other substances (organic impurities, metal sulfates, etc.) or as by-product (CaSO.sub.4).
The recovery of sulfuric acid from the processes which are used by such industries presents a problem which has not yet found a convenient solution.
At present, various methods are used for the recovery of the sulfuric acid which originates from the waste acids of such processes, such as:
(1) The regeneration of waste sulfuric acid by decomposition at high temperature (.about.1000.degree. C.) to SO.sub.2, reoxidation to SO.sub.3, and absorption in water. The regenerated acid contains 93-95% H.sub.2 SO.sub.4 (Sander, U. and Daradimas, G., Chem. Ing. Progr., Sept. 1978; Regenerating Spent Acid; p. 57-67).
(2) The Simonson-Mantius process (Burke, J. F., and E. Mantius, Chem. Eng. Progr., 43,237 (1947); Duecker, W. W., and J. R. West, eds., "The Manufacture of Sulfuric Acid", p. 238, Reinhold, N.Y. (1959) which recovers waste sulfuric acid by evaporation under vacuum.
(3) The Chemico process (G. M. Smith and E. Mantius, Chem. Eng. Progr. pp. 78-83, (Sept. 1978) which concentrates the waste sulfuric acid by utilizing hot gases.
The disadvantages which are common to these recovery processes are the large quantity of energy consumed and the corrosion which is caused by the high concentration of regenerated acid.
In the case where the recovery of sulfuric acid is not profitable because of the low concentration of H.sub.2 SO.sub.4 in the waste acid or because of the high percentage of impurities, lime or limestone are used to neutralize the waste acid. The CaSO.sub.4 obtained by neutralization often contains impurities which make it unacceptable as gypsum. In this case, the CaSO.sub.4 presents a problem because it has to be eliminated.
For the large majority of industries which use sulfuric acid in their processes, the disposal of waste acid constitutes a serious problem because of the regulations governing environmental protection. By way of example, an important process in the pigment industry uses sulfuric acid in order to solubilize TiO.sub.2 from raw material. At the end of this process a solution is rejected which contains mainly 250-350 g/l H.sub.2 SO.sub.4 and, for example, .about.7 g/l TiO.sub.2, .about.3 g/l VOSO.sub.4 and 22 g/l Al.sub.2 (SO.sub.4).sub.3. This solution is discharged directly to a river, constituting a serious source of pollution.
Another example is the process for leaching calcined zinc sulfide with sulfuric acid and electrolyzing the sulfate solution to recover zinc. An important waste solution derived from this process contains about 250 g/l H.sub.2 SO.sub.4, 13 g/l dissolved Zn and 10 g/l dissolved Mn. These examples are not exhaustive.